1. Field of the Invention
The present invention is directed to a balun transformer for providing a single ended output signal from a pair of differential input signals, and more particularly to a transmission line balun implemented by a pair of inter-coupled transmission line signal couplers.
2. Description of the Related Art
As is well known, RF wireless circuits utilize balanced outputs of signals to minimize the effect of ground inductance and to improve common mode rejection. Such circuitry include mixers, modulators, IF strips and voltage controlled oscillators. These balanced outputs, moreover, consist of differential signals which must be combined to provide a single ended output signal. One known type of device for combining differential signals into a single ended output signal is referred to in the art as a "balun" (balanced input/unbalanced output). Typically, baluns are tightly coupled structures fabricated much like a conventional transformer utilizing discrete components; however, the turns are arranged physically to include the interwinding capacitances as components of the characteristic impedance of a transmission line. Such a technique can result in increasing the bandwidth of the device up into the megahertz frequency range. More Recently, baluns have been implemented using distributed components. When implemented with discrete components, they add excessive loss and increase the cost of fabrication. When implemented in distributed form they exhibit less loss, but at wireless frequencies require a relatively large amount of board space together with an inherent limitation of being narrow band devices.